What are IN & OUT instructions in x86 used for?

I've encoutered these to instructions IN & OUT while reading "Understanding Linux Kernel" book. I've looked up reference manual.

5.1.9 I/O Instructions

These instructions move data between the processor’s I/O ports and a register or memory.

IN    Read from a port
OUT   Write to a port
INS/INSB  Input string from port/Input byte string from port 
INS/INSW  Input string from port/Input word string from port 
INS/INSD  Input string from port/Input doubleword string from port
OUTS/OUTSB    Output string to port/Output byte string to port 
OUTS/OUTSW    Output string to port/Output word string to port 
OUTS/OUTSD    Output string to port/Output doubleword string to port

I didn't get few things:

1. "processor’s I/O ports". What are they? Why would we want to read & write "strings" to & from these ports?
2. I never encoutered a scenerio where I need to use these instructions. When would I be needing these?
3. Give some practical examples.